1. Field of the Invention
The present invention relates to an optical transceiver and a projection covering member. In particular, the present invention relates to a technique for suppressing electromagnetic interference (EMI).
2. Description of the Related Art
In recent years, the optical transmission speed of an optical transceiver has changed from 2.5 Gbit/s to 10 Gbit/s and further to 40 Gbit/s. Therefore, an internal clock frequency of a circuit used for optical communication or the like has also increased to be from several hundred MHz to several tens of GHz. In connection with that, the frequency of the EMI has also increased to be several tens of GHz. In particular, it is an urgent necessity to establish a technique for suppressing the EMI at the upper limit frequency of 40 GHz and its vicinity because of a Federal Communications Commission (FCC) regulation that is an EMI regulation in the U.S.A.
In addition, to respond to a request for improving mounting efficiency of the optical transceiver that has been compact and downsized, a so-called pigtail type has become a common structure of the optical transceiver and in which pigtail type, a projection, which includes an optical coupler of an optical subassembly, protrudes to the outside of a case.
However, with respect to such a pigtail type optical transceiver, electromagnetic wave or noise current may occur in the printed circuit board or in the optical subassembly and may propagate through the projection protruding to the outside of the case so as to reach the outside of the case, which becomes a factor in generating the EMI.
Concerning this point, JP 2003-304207 A (hereinafter referred to as “Patent Document 1”) and JP 2002-365491 A (hereinafter referred to as “Patent Document 2”) disclose techniques for suppressing the EMI radiated from such a pigtail part of the optical transceiver.
Specifically, Patent Document 1 discloses an optical module having a metal cap that contacts with a lower case part and an upper case part or is integral with the case so as to further cover the protruding part of a projection which includes an optical coupler covered with an elastic member. In addition, Patent Document 2 discloses an optical module having a projection which includes an optical coupler covered with an electromagnetic wave absorber.
However, the optical module described in Patent Document 1 only shields the EMI radiated from the protruding part of the projection covered with the elastic member on the outside thereof, but it cannot provide an effect of attenuating the noise current (noise magnetic field) generated in the projection. Therefore, there is a problem that the electromagnetic wave may leak from a slight gap in the case.
In addition, the electromagnetic wave absorber described in Patent Document 2 cannot provide a sufficient effect of absorbing the electromagnetic wave of the EMI having a frequency of approximately several tens of GHz to 40 GHz generated by the optical subassembly. In addition, there is a problem that the effect of suppressing the EMI generated by other components constituting the optical module is also insufficient.